In the U.S., Latino women (Latinas) report higher rates of inactivity than their non-Hispanic White and male counterparts and thus are disproportionately burdened by related health conditions (e.g., cancer, hypertension, heart disease, stroke, diabetes). Cultural factors, socioeconomic circumstances, language and educational barriers limit Latinas access to public health interventions that promote physically active lifestyles. To address this public health crisis, effective interventions that leverage state-of-the-art theory and methods are needed to reach this at-risk population. Our research group has over 20 years experience developing and evaluating individually-tailored, computer expert system-driven, physical activity interventions (based on Social Cognitive Theory and the Transtheoretical Model) through various channels and settings. In our recent pilot (R21NR009864), we culturally and linguistically adapted our tailored intervention for sedentary Latinas and conducted a small randomized trial of the modified program (N=93). A total of 81 participants completed the 6- month study (87 percent retention) and increased their physical activity from a mean of 17 minutes per week (SD=25.76) at baseline to 147 minutes (SD=241.55) at six months whereas contact control participants increased their physical activity from 12 minutes per week (SD=21.99) at baseline to 97 minutes (SD=118.49) at six months. These observed improvements in physical activity in our intervention group, along with high retention rates, participant-reported desire for an Internet-delivered program, and formative work developing our program in an Internet format lend support for testing an Internet intervention tailored to the needs of Latinas. Therefore, for the current application, we will build on our previous work by conducting an adequately powered (N=200) randomized controlled trial to test the efficacy of our culturally and linguistically modified, individually tailored physical activity intervention delivered via the Internet relative to an Internet wellness contact control condition (including cardiovascular health information developed for Latinos by the NHLBI). Data will be collected at baseline, 6 months (post-treatment), and 12 months (maintenance) using well- established physical activity measures (7-Day PAR, Actigraphs), as well as a comprehensive set of psychosocial questionnaires. We hypothesize that at end of treatment (month six) intervention participants will report significantly more minutes of moderate intensity physical activity per week than the wellness contact control participants. We will also examine the maintenance of treatment effects at 12 months, potential mediators and moderators of the intervention-physical activity relationship, and the costs of delivering the tailored Internet program. In the proposed study we seek to promote physical activity among an underserved population using a high-reach, low-cost, technology-based strategy, which has great potential for adoption on a larger scale and thus high potential for reducing health disparities in the U.S.